Anode/Cathode

tinylilron said:

So, while studying for the MCAT today I just learned that anode and cathode sometimes switch charges. I always thought that the anode was negatively charged and the cathode was postively charged but I was wrong. Apparently this is only the case in a Galvanic Cell. In a Electrolytic Cell (used in Electrophoresis) this is exactly the opposite where the anode is positively charged and the cathode is negatively charged.

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This is a very common mistake, and often messes people up when they go from electrical circuits to electrophoresis. In a DISCHARGING system such as a battery driven circuit, the cathode of the battery is positive and the anode of the battery is negative. However, in a CHARGING system such as a capacitor filled with a gel used in electrophoresis, the electrodes are defined in an oppisite fashion. The anode has a positive charged forced upon it by the applied voltage and the cathode has a negative charge forced upon it by the applied voltage. Just keep an eye out for charging versus discharging, and you'll be fine.

As for an electrolytic cell having it's polls defined differently, that is NOT true. They are reversed from the galvanic version of the cell, but the anode is still negative and the cathode is still positive (by convention). In reality, the plates are actually not charged at all. The anode is the site of oxidation, so it generates the electrons. The cathode is the site of reduction, so it absorbs the electrons. You can think of anodes as electron rich and cathodes as electron poor.

tinylilron said:

Apparently this is because: "An anode is an electrode through which electric current flows into a polarized electrical device." And the cathode is the opposite. What does that mean in the above paragraph? I am having trouble understanding that.

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You're not the only one having trouble with that sentence. I teach this stuff at the college level and I can't figure out what they are trying to say. There must be more to that paragraph, because by itself, that sentence tells us nothing.

tinylilron said:

If someone could explain it to me in simple terms that would be wonderful. In addition, what would be the best way to memorize this/understand this concept for MCATs. Does the MCATs use both types of cells in which the anode/cathode charges switch?

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If you have a friend who happens to have the BR chemistry books, read pages 270-271 of Book 2. That should explain it in a very simplified way that will work for any cell on the MCAT. It's basically something you should know for redox in terms of electron rich and electron poor, not necessarily charged in one way or another. It makes it MUCH easier to think about.